1. Field of the Invention
The present invention relates to a billet used in precast forming of metal. The present invention also relates to a horizontal continuous casting process which cools molten metal continuously and horizontally draws out a solidified cast piece, and in particular, relates to a horizontal continuous casting process which is effective in the case of using hypo-eutectic cast iron. The present invention relates to a thixocasting process which performs pressure casting using the above billet and in particular, relates to a thixocasting process which prevents an oxide film formed on the surface of a billet from entering into the billet at a low production cost
2. Description of the Related Art
Continuous casting processes have been widely used as processes for mass-producing uniform and high quality metal material at low cost. The continuous casting processes include a vertical type process in which a cast piece is drawn out downwardly and a horizontal type process in which a cast piece is drawn out horizontally, and the horizontal type process is employed more often than the vertical type process in view of lower equipment cost. In the horizontal type continuous casting process, generally, molten metal stored in a tundish is supplied into a mold which is horizontally installed and is simultaneously cooled, and a cast piece in which at least the circumference portion is solidified in the mold is thereby formed, and then, the cast piece discharged from the mold is continuously and horizontally drawn out by drawing out equipment.
The above mold used in the horizontal continuous casting is of a cylindrical shape or a prism shape and is provided with a cooling jacket at the circumference thereof. Therefore, the mold acts so that a solidified shell grows by continuously supplying molten metal into the inside and by cooling, and a forming position of the solidified shell, that is, a solidifying initiation position of molten metal is stabilized. Materials of the mold generally differ between the case in which the cast is cast iron and in the case in which it is steel for the following reasons.
Since the cast iron has relatively low toughness, cracks, which are a type of surface defect which is easily generated, and breakouts or fractures of cast pieces which are easily generated, occur when friction between the cast piece and inner wall surface of a mold is high, and therefore, graphite having superior lubricity is used therewith. Here, the term “breakout” refers to a deficiency in which cracks are generated on the surface of a cast piece discharged from a mold and the cracks reach the interior non-solidified portion by extending, and molten metal leaks or erupts, and the term “fracture” refers to a state in which a cast piece is cut off after perfectly solidifying the inside. When a breakout or fracture is generated, the drawing out of the cast piece must be stopped. Since the cast iron has relatively low solidifying contraction, it is difficult to generate a gap between the cast iron and mold by the solidifying contraction, and therefore, a solidified shell can be efficiently grown by cooling when a long mold made of graphite is provided. In continuous casting of the cast iron, a solidified shell may be grown by carrying out secondary cooling in which air is blown or water mist is sprayed just after discharging from the mold.
In contrast, in the case in which the cast is of steel, a mold made of graphite is easily damaged by molten metal. When the damage by molten metal occurs, surface quality of the cast is deteriorated, and C (carbon) of the mold damaged by molten metal permeates into the steel and the amount of C in the cast piece is thereby increased. Therefore, a mold made of a Cu alloy is employed. Since the steel has relatively large solidifying contraction, it is easy to generate a gap between the steel and mold by the solidifying contraction, and in particular, in horizontal continuous casting, generation of the gap shifts to the upper side of the mold due to gravity. According to the generation of the gap, coolability of the cast piece to be cooled by contacting the mold is significantly decreased. Thus, it is proposed that a solidified shell of a cast piece be grown by supplying molten metal into a fixed first mold, and then the cast piece be passed to a second mold which can move in a radial direction, and the gap is eliminated by pressing the cast piece by the second mold. This second mold is well known, for example, from Japanese Utility Unexamined Publication No. 5-93641. In horizontal continuous casting combined such a first mold and a second mold, the first mold has a length of 200 mm or more. Additionally, the cast piece is intermittently drawn out generally in strokes of 40 to 50 mm.
The reasons for intermittently drawing out the cast piece are as follows. The mold has a temperature gradient in which the temperature gradually decreases from the tundish side toward the drawing out direction. When the cast piece is continuously drawn out, the temperature of the molten metal passes a solidifying initiation temperature according to the temperature gradient; however, in this case, the solidification interface is easily disturbed by uneven temperature, or the like. In contrast, when the cast piece is intermittently drawn out, the temperature of the molten metal passes a solidifying initiation temperature at a cooling rate above the temperature gradient of the mold, and the cast piece is solidified rapidly. Therefore, the solidification interface is stably formed, and a sound cast piece can be thereby cast.
Incidentally, a continuous casting material made of a hypo-eutectic cast iron has recently attracted attention, as a good machinability cast iron or material for a half-melted molding, having a high Young's modulus or high strength. However, the growth of a solidified shell is slow since the hypo-eutectic cast iron has a wider temperature range of solid-liquid phase coexistence than that of a cast iron or steel, and therefore, cracks are easily generated in the solidified shell, and moreover, a half-solidified structure having decreased flowability frequently prevents molten metal from being supplied. In addition, the cast piece has low toughness and cracks are easily generated in the solidified shell, since the solidified shell is easily cooled. Furthermore, because solidification contraction is relatively large, a gap easily forms between the cast piece and the mold, and efficient growth of the solidified shell cannot be as desired. From these reasons, breakouts or fractures easily occur and it is difficult to carry out continuous casting, even if the above mobile second mold is used, and therefore, development of an effective continuous casting process has been desired.
In addition, a billet as a material for casting using a thixocasting process forms an iron oxide film on the surface thereof when it is heated in a half-melted state in the air. This oxide film contributes to the form maintaining property of the billet in a half-melted state; however, when the billet is transformed in heating or in inserting the billet into a sleeve, the oxide film often permeates the inside of the billet as foreign material in the subsequent injection molding, and consequently, a reduction of the product strength occurs.
In order to overcome the above deficiencies, so far, for example, as described in Japanese Patent Unexamined Publication No. 5-42352, a surface decarbonization film layer was formed by previously decarburizing the surface of billet and a property of the billet in a half-melted state was improved, and desired product strength was thereby obtained.
However, it is necessary to carry out a process in which heating at 700 to 1000° C. for over 20 minutes in air or in which heating at 700 to 1200° C. for over 10 minutes in a reducing atmosphere including water in order to form the surface decarbonization film layer, and a desired low production cost could not be realized. For this reason, development of a billet for thixocasting which can prevent an oxide film from permeating to the inside of the billet in injection molding at low cost, and a thixocasting process which is carried out by pressure-casting using the billet, have been desired.